Problems of smudging and record media damage in the nature of tearing and snagging have been experienced in connection with the use of impact printers, especially that type of impact printer which employs reciprocating movement of one or more relatively long, thin wire-like printing elements, sometimes called dot matrix printers. This is due at least in part to the fact that such printing elements move only a very short distance, usually not more than 0.015 inches, in a printing excursion, and consequently the medium to be printed upon must be positioned in very close proximity to the printing elements and the ink ribbon.
The smudging and record media damage problems have been recognized in the past, and proposed solutions have included the use of ribbon masks interposed between the ribbon and the record media being printed upon. In one arrangement, a flexible mask has been added to the cassette which holds the ink ribbon. Another solution involves the use of a ribbon mask which forms part of the structure of the printing apparatus, such as is shown in U.S. Pat. No. 4,165,188. These arrangements would appear to be useful primarily in the feeding of a platen means for supporting record media for printing; printing means comprising a plurality of impact printing elements and a frame for mounting said printing elements for reciprocating printing movement; means for maintaining a predetermined distance between said platen means and said printing means during a printing operation; ribbon guide means secured to said printing means for guiding a printing ribbon into cooperative relation between the ends of said print elements and the platen means on which record media may be positioned for printing; ribbon mask support means secured to said printing means; ribbon mask means secured adjacent one end thereof to said ribbon mask support means and slidingly coupled to said support means adjacent the other end of said ribbon mask means, said ribbon mask means being positioned to receive the printing ribbon between said mask means and the ribbon guide means, and having an aperture therein to accommodate movement therethrough by the printing elements; a cam surface on the ribbon mask support means comprising an angled extension thereof; and a cam surface on the platen means comprising a planar element secured at an end of the platen means and disposed at an angle to the reciprocating direction of movement of the platen means for cooperative engagement with the angled extension of the ribbon mask support means to increase the predetermined distance between the printing means and the platen means at the end of a printing line, whereby printing ribbon insertion is facilitated.
In order to overcome the conditions of smudging of record media by ink ribbons and document damage by tearing at a thickness transition, a ribbon mask must possess sufficient stiffness to move over the media thickness transition without collapsing against the ribbon and thereby causing undesirable drag and loading effects on the ribbon drive mechanism. At the same time, the relatively small clearance between the print head and the record media must not be exceeded, so that proper printing can continue to take place. Means must be provided for adequate clearance between the print head and the ink ribbon mask when necessary to enable ribbon insertion and removal. In addition, the opening in the mask through which the print elements impact the media must be of the proper configuration and must not present an edge to the record media which would result in snagging and tearing.